In a fuel injection system for a diesel-powered engine, fuel at high pressure is injected rapidly, and usually directly, into the fuel combustion chamber. Inside the combustion chamber the fuel mixes with air and combusts with the available oxygen to generate the power that is used to drive the vehicle.
One of the advantages of current fuel injection systems is that the amount of fuel entering the combustion chamber can be precisely controlled so as to improve the performance and emissions of the engine. For instance, better emissions can be achieved by matching the amount of fuel entering the combustion chamber in response to a signal indicating the amount of air flowing in to the engine.
In order to optimise the emissions performance of an engine (e.g. minimise NOx and particulates), extensive calibration and testing is performed on an engine, typically during engine development, under controlled conditions (for example, known fuel composition and fuel temperature). Hence, information such as: the optimum amount of fuel required to match a certain volume of air from a turbo charger, or to control the temperature of an after-treatment system such as a diesel particulate filter (DPF), is typically found by experimentation and programmed into an engine management computer or engine control unit (ECU).
However, it is known that the physical properties of a fuel (e.g. the viscosity, isoentropic bulk modulus and density) can vary quite considerably according to factors such as temperature and fuel composition. These properties in turn affect how the fuel behaves in a fuel injection system: for example, how quickly it flows; what its pressure is; and consequently, how much fuel is injected at each fuel injection event (stroke). This variation can be particularly significant for diesel engines (as opposed to gasoline engines), because diesel engines can often run on a relatively wide variety of fuel compositions, including biodiesels and biodiesel blends. By way of example, it is not uncommon for commercially available diesel fuel to have different compositions in winter and summer for optimal performance in different extremes of ambient temperature.
Biodiesel is considered to be an environmentally friendly alternative to traditional fuels such as diesel. It is derived from renewable resources such as vegetable and animal fats and waste cooking greases, and importantly, it can be mixed with petroleum-based diesel to create a blend, without causing damage to a diesel engine. Due to its wide variety of possible compositions, biodiesel can cause changes in engine performance and emissions. For example, Tat, M. E. & Van Gerpen, J. H. (ASAE paper no. 026084, ASAE Annual International Meeting, USA, Jul. 28-31, 2002) reported an approximately linear correlation between the biodiesel blend composition (i.e. the proportion of biodiesel and diesel in a blend) and the physical properties of density, isoentropic bulk modulus and the speed of sound transmission within the fuel.
Variations in the physical properties of a fuel can significantly impact on the actual amount of fuel that is injected into a combustion chamber in a single fuel injection event. When it is then considered that there could be over 70 injection events into the engine every second, the actual amount of fuel injected in any period of time could be considerably higher or lower than the actual fuel demand. Such a difference between fuel supply and demand can cause an engine to operate considerably differently to when it was first calibrated, with a consequential reduction in engine performance, perhaps in terms of one or more of power output, fuel economy and engine emissions.
Prior art fuel injection systems have fuel temperature sensors that use a thermocouple to measure fuel temperature. However these are both an additional expense and a potential point of failure. Moreover, such temperature sensors only measure the temperature in the vicinity of the sensor, and not across the bulk of the body of fuel. Composition sensors also exist that measure gasoline/ethanol blends (see Tat, M. E. and Van Gerpen, J. H. supra.). Again these are separate components, which introduce additional expense and potential point of failure issues.
Therefore, in order to accurately control a fuel injection system it would be advantageous to be able to mathematically model the behaviour of fuel under any set of conditions. For this reason, it would be beneficial for a fuel injection system to be able to estimate the composition of fuel within the engine/system and then compensate for its specific physical properties to optimise fuel injection events, for example, at engine start-up. Also it would be advantageous to estimate (or calculate) the temperature of fuel within the engine/system and then compensate for any changes in the physical properties of the fuel as the temperature of the fuel changes during engine use, i.e. under non-ambient conditions. Advantageously, the methods of the system operate without the need for additional temperature/composition sensors: for example, by using existing pressure sensors.
Accordingly, there is a need for a method of estimating the physical properties of fuel in an engine in real time, and under both ambient and non-ambient (i.e. operating) conditions. In addition, there is a need for an improved fuel injection system that is capable of responding to changes in fuel physical properties, engine conditions and/or fuel composition so as to accurately satisfy a specific (or change in) fuel demand under operating conditions.
This invention aims to overcome or alleviate some of the problems associated with the prior art.